Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices and more particularly to a means for adjusting center frequency by deposition of a nonconducting film.
Surface acoustic waves-also known as acoustic waves, surface elastic waves or elastic surface waves-are elastic waves which propagate along a material surface or interface and whose amplitude decreases with the normal distance from said surface or interface. The best-known form of surface wave is termed the "Rayleigh wave", but there exist other modes of elastic wave propagation which can be included in the category of surface acoustic waves. A SAW device, then, is any device making use of surface acoustic waves.
SAW devices generally consist of a number of metal interdigitated electrodes deposited on a piezoelectric substrate, though other configurations are sometimes used. SAW devices are used in the generation and analog processing of electronic signals in the range of 10.sup.7 -10.sup.9 Hz. A problem that has arisen is that users often specify the center frequency of the passband of narrow-band SAW devices to within 100 ppm or less, while with the present technology accuracy better than 0.5% is difficult and expensive to obtain. Some tuning of SAW devices is possible by means of adjusting the other electronic circuit components values (such as inductance, resistance, capacitance), but such external tuning is ordinarily usable over only a very limited range. This problem has led to efforts to adjust the center frequency of SAW devices after fabrication. One means of adjusting the center frequency is by reducing the thickness of the metal electrodes by chemical etching. Etching allows adjustment in only one direction by a very small amount and is difficult to monitor precisely. Alternatively, the frequency can be adjusted by the use of a laser trimmer to burn off electrode fingers or portions of electrode fingers. This method requires ulta-precise and expensive laser equipment and necessitates the use of special electrode designs to obtain both upward and downward adjustment capability.